As far as I am aware, for a graphics card to work in a Mac as the primary card it needs an EFI compatible ROM.. Perhaps that may have changed, but the last time I looked into this that was the case.

Now, I do recall reading something about using gfx cards with BIOS compatible ROMs as secondary cards and getting it dragged in on the coattails of the primary card that is EFI compatible.... but I never looked much into that part.